Compression-type refrigerators are generally constituted with a compressor, a condenser, an expansion valve and an evaporator, and has a structure in which a mixed fluid of a refrigerant and a lubricating oil is circulated in the closed system. Heretofore, in the compression-type refrigerators, dichlorofluoromethane (R12), chlorodifluoromethane (R22) or the like has mainly been used as the refrigerant, and various types of mineral oil and synthetic oil have been used as the lubricant.
However, chlorofluorohydrocarbons, such as R12 and R22 described above, are being more rigorously restricted world-wide because they bring environmental pollution such as the ozonosphere destruction. By this reason, hydrogen-containing Flon compounds [a "Flon compound" means a chlorofluorocarbon, a hydrofluorocarbon, and a hydrochlorofluorocarbon in general] such as hydrofluorocarbons and hydrochlorofluorocarbons are attracting attention as the novel types of refrigerant. The hydrogen-containing fluorocarbons, particularly hydrofluorocarbons, such as 1,1,1,2-tetrafluoroethane (Flon 134a), are preferred as the refrigerant for compression-type refrigerators because they have little possibility of causing the ozonosphere destruction and can replace Flon 12 with little change in the structure of refrigerators which have heretofore been used.
However, when the hydrofluorocarbon refrigerant described above is used, an unfavorable phenomenon that foaming takes place to a great extent during boiling of the refrigerant dissolved in the refrigerator oil occurs. The foaming phenomenon takes place to a greater extent particularly when a mixed refrigerant containing two or more types of hydrofluorocarbon is used. When the foaming phenomenon occurs to a great extent, a large amount of the refrigerator oil flows into the refrigeration system to cause problems that the refrigerating ability is decreased and that the sufficient lubrication is not achieved because of decrease in the amount of the refrigerator oil at the places requiring lubrication. Particularly when a refrigerator of a recent type equipped with an inverter is used, the problems are greater because a higher speed of rotation is required at the start of the operation. Therefore, a refrigerator oil which can prevent the foaming phenomenon is required more for a refrigerator of this type than for refrigerators of previous types.
The same phenomenon has also been observed with refrigerants of previous types. It has empirically been known that the foaming phenomenon in the refrigerants of previous types can be suppressed by adding a conventional silicone oil. However, it is the current situation that, when a hydrofluorocarbon refrigerant is used, the addition of a conventional silicone shows such limited effect of suppressing the foaming phenomenon that no practical effect can be expected.